What Are the Different Types of Twins?

Identical twins don’t always look alike, and multiples can be identical, fraternal — or both. Here’s everything you need to know about different types of twins and multiples.

What's better than snuggling up with your baby bundle? How about snuggling up with two (or more!) bundles at the same time? Whether you're pregnant with multiples — or you know someone who is — they're more common than ever. In fact, the rate of twin births in 2009 is 76 percent higher than it was in 1980, according to experts. But not all twins are born alike. Some are fraternal, and others are identical. So what's the difference, anyway? Here's the scoop on the various types of twins and multiples.

Identical Twins vs. Fraternal Twins

Identical twins, also called monozygotic twins, develop from a single fertilized egg. Depending on when that egg splits, they might also share the same placenta. They are much less common than fraternal twins.

Identical twins are genetic duplicates, meaning traits like sex, eye color and hair color will always be the same. But even though these twins have the same DNA, certain environmental factors, like placement in the womb, can affect each baby's appearance, making them look slightly different from each other. Some twins' heads, for example, might not be shaped in exactly the same way. Their hair might be different, too. Other traits, such as personalities and allergies, can also diverge. One twin, for example, might have a severe peanut allergy, while the other doesn't.

Because they don’t always appear to be carbon copies of each other, identical twins can be confused with fraternal twins, also called dizygotic twins, which develop when two eggs are fertilized separately — often because a mother has released two eggs at the same time during ovulation. Fraternal twins don’t have identical DNA and are no more genetically similar than any other siblings. They can be opposite sexes and can look completely different from each other.

The rising rate of multiple births is associated with an increase in fraternal twins — and the growing number of older moms. As a woman ages, her chance of having twins jumps because she’s more likely to release more than one egg at a time when she ovulates. That coupled with the more frequent use of ovulation-inducing drugs and in vitro fertilization (IVF) have contributed to the surge in fraternal twins.

Other Types of Multiples

Higher-order multiples like triplets, quadruplets and quintuplets are less common than twins of either kind. They can be identical, fraternal — or even both. For example, if an egg splits and then splits again, you can have quadruplets. If this happened at the same time that a second egg was released, then you might end up with a mixture of identical and fraternal quintuplets.

You’ve probably met at least a few sets of fraternal twins because they’re so common and maybe even some identical twins too. But you’re much less likely to have encountered these rarer types of multiples:

Mirror image twins. In about 23 percent of identical twins cases, the fertilized egg splits later than usual, creating mirror image twins. Just as the name implies, these babies often have characteristics that mirror each other: Their birthmarks and other physical traits will be exact opposites. For instance, when one twin has a birthmark on the right side of her head, the other twin will have the same birthmark on the left side.

Superfetation. In exceedingly rare cases, a woman might release two eggs during ovulation, but not at the same time. In such cases, twins may actually be born on different days — even weeks or months apart.

How Can You Tell Whether Your Twins Are Identical or Fraternal?

The only way to find out whether your multiples are fraternal or identical is to have them take a DNA test, which consists of a simple swab on the inside of the cheek. The screening is virtually 100 percent accurate. A twin zygosity test compares the DNA profiles of each twin to see if they match. Some parents choose to have their babies screened at birth, and others wait until their twins are toddlers if they're curious and want to know for sure.

A growing number of parents are finding out that they're expecting not one, but two or more little ones. It's both exciting and chaotic all at the same time. Multiples are definitely their own kind of challenge, but don't panic if you're pregnant with more than one baby! You can always keep up your stamina with some double-duty cuddle sessions after they're born.

From the What to Expect editorial team and Heidi Murkoff, author of What to Expect When You're Expecting. Health information on this site is based on peer-reviewed medical journals and highly respected health organizations and institutions including ACOG (American College of Obstetricians and Gynecologists), CDC (Centers for Disease Control and Prevention) and AAP (American Academy of Pediatrics), as well as the What to Expect books by Heidi Murkoff.